Awesome link. Its interesting to contrast the quotes from person who appears to have been responsible for escalation and then the person who put on the brakes.

Dave Eshelman was a prison guard:

What came over me was not an accident. It was planned. I set out with a definite plan in mind, to try to force the action, force something to happen, so that the researchers would have something to work with. After all, what could they possibly learn from guys sitting around like it was a country club? So I consciously created this persona. I was in all kinds of drama productions in high school and college. It was something I was very familiar with: to take on another personality before you step out on the stage. I was kind of running my own experiment in there, by saying, "How far can I push these things and how much abuse will these people take before they say, 'knock it off?'" But the other guards didn't stop me. They seemed to join in. They were taking my lead. Not a single guard said, "I don't think we should do this."

The fact that I ramped up the intimidation and the mental abuse without any real sense as to whether I was hurting anybody— I definitely regret that. But in the long run, no one suffered any lasting damage. When the Abu Ghraib scandal broke, my first reaction was, this is so familiar to me. I knew exactly what was going on. I could picture myself in the middle of that and watching it spin out of control. When you have little or no supervision as to what you're doing, and no one steps in and says, "Hey, you can't do this"—things just keep escalating.

Christina Maslach was dating Zimbardo (and later married him) and visited the experiment while it was in progress:

At first Phil didn't seem different. I didn't see any change in him until I actually went down to the basement and saw the prison. I met one guard who seemed nice and sweet and charming, and then I saw him in the yard later and I thought, "Oh my God, what happened here?" I saw the prisoners being marched to go down to the men's room. I was getting sick to my stomach, physically ill. I said, "I can't watch this." But no one else was having the same problem.

Phil came after me and said, "What's the matter with you?" That's when I had this feeling like, "I don't know you. How can you not see this?" It felt like we were standing on two different cliffs across a chasm. If we had not been dating before then, if he were just another faculty member and this happened, I might have said, "I'm sorry, I'm out of here" and just left. But because this was someone I was growing to like a lot, I thought that I had to figure this out. So I kept at it. I fought back, and ended up having a huge argument with him. I don't think we've ever had an argument quite like that since then.

I feared that if the study went on, he would become someone I no longer cared for, no longer loved, no longer respected. It's an interesting question: Suppose he kept going, what would I have done? I honestly don't know...

People will sometimes come up to me—at conferences, or maybe they're students who have taken psychology classes—and they'll say, "Oh my God, you're such a hero! What is it like to be a hero?" And it's always a little surprising to me because it sure didn't feel heroic at the time. The prison study has given me a new understanding of what "heroism" means. It's not some egocentric, I'm-going-to-rush-into-that-burning-building thing—it's about seeing something that needs to be addressed and saying, I need to help and do something to make it better.

I felt that throughout the experiment, he knew what he wanted and then tried to shape the experiment—by how it was constructed, and how it played out—to fit the conclusion that he had already worked out.

From John Mark, one of the day guards.

A follow-up study in 2007 should not surprise anyone who knows a bit of psychology: when you ask for students to participate in a study about prison life, you get a different subset of people.